Polymer compounding is a well-known process for preparing plastic formulations by the mixing and/or blending of polymers, such as polyamides or polyesters, with, for example, additives, fillers, reinforcing agents and modifiers while the polymer is in a melted or semi-melted state. See, e.g., U.S. Pat. Nos. 6,149,850; and 5,236,652. Compounding typically takes place in an extruder. The step(s) of mixing and/or blending with, for example, additives, fillers, reinforcing agents and modifiers generally occurs prior to the shaping step, i.e., prior to extrusion, injection molding, etc. The additives, fillers, reinforcing agents and modifiers impart preferred properties to the host polymer to which they are added.
It is desirable that the final plastic product contains as low as possible amount of volatile components such as residual monomers, organic solvents and water. The presence of significant levels of water promotes depolymerization (hydrolysis) and reduces the molecular weight and melt viscosity of the polymer to the point where bubbles form in the extrudate. Degassing of the polymer during the compounding process is the typical means for achieving a low volatile component content. To achieve effective degassing of the volatile components during the compounding process, high vacuum and high temperatures are required, but at high temperatures, degradation of the polymer significantly increases. If, however, temperatures in the extruder are maintained at levels to minimize degradation of the polymer, effective melting and blending of the compound components is negatively impacted, which has a deleterious effect on the quality of the final product. See, e.g., U.S. Pat. No. 8,034,269. These problems are particularly prevalent at high throughput rates, where the short exposure time of the polymer to the compounding process is dictated by cost efficiency. Thus, at a high throughput rate, increasingly effective degassing of the volatile components occurs at the expense of an increase in the rate of degradation of the polymer. An economic analysis is required to determine the acceptable balance between these two competing outcomes. It is known, for example, that polyamide resins useful in molding and extrusion applications experience undesirable mold buildup over time, thereby reducing machine uptime and capacity utilization. See, e.g., Plastics Technology (2000) (http://www.ptonline.com/articles/engineering-thermoplastics); page 13 of the DuPont™ ZYTEL® HTN Molding Guide (2001) (http://www.dupont.com/content/dam/assets/products-and-services/plastics-polymers-resins/Documents/H85940.pdf which acknowledge the problems associated with vent plugging. U.S. Pat. No. 6,518,341 refers to vent accumulation, how it relates to burning in the mold and introduces a “shots before burn” term which relates to mold deposits or plate-out and how the productivity of injection molding is adversely impacted by this mold depositing.
Polyamide resins are routinely compounded by use of an extruder for mixing, for example, additives, fillers, reinforcing agents and modifiers with polyamides such as nylon, including nylon 6, nylon 4/6, nylon 6/6, nylon 6/10, nylon 6/12, nylon 11 and nylon 12, where the polyamide is in a solid (e.g., pelletized or flake or chip) form. Typically, the compounding process comprises introducing nylon feedstock in solid form into the first half of an extruder to achieve melting of the feedstock. After melting has occurred, one or more additives, fillers, reinforcing agents and modifiers are introduced and mixed into the nylon melt. In an extruder containing multiple barrel zones, a vacuum is typically applied to remove undesired volatiles and moisture from the polyamide and the additive, filler, reinforcing agent and/or modifier stream. If not removed, these undesired components foul the customer's mold over time, thereby reducing machine uptime and capacity utilization. See, e.g., A. Dreiblatt, Plastics Technology (2010) (http://www.ptonline.com/columns/in-twin-screw-compounding-distinguish-between-disease-and-illness). U.S. Pat. No. 6,518,341 also describes the problems resulting from the presence of deposits in injection molding operations.
Accordingly, longer exposure time of the polyamide melt to vacuum conditions and to a higher vacuum improves the product quality by decreasing the level of water and any residual impurities such as low molecular weight oligomer content such as nylon oligomer, monomers, stearates and wax components. The removal of impurities is limited by the number of extruder barrel sections that can be used for vacuum exposure, i.e., those barrels not already utilized for nylon melting and glass or additive feed points. The residual content of volatile components is typically determined by means of gas chromatography, with quantitative evaluation carried out using an internal standard.
It is known that variability in the feedstock significantly contributes to poor process stability and lack of quality control for the compounded product. See, e.g., Extrusion: The Definitive Processing Guide and Handbook (2005) by Harold F. Giles Jr. et al. Particularly when the feedstock is stored in a solid form, this undesired variability originates from multiple sources (e.g., differences in moisture levels between samples and the age of the feedstock). As a result, drying of the solid feedstock is frequently required prior to extrusion to achieve a controlled and consistent moisture content necessary to prevent significant reduction of relative viscosity (RV) due to hydrolysis. See, e.g., Extrusion: The Definitive Processing Guide and Handbook (2005) by Harold F. Giles Jr. et al.
Poor warehouse management further contributes to the variability observed in the precursor compounding operation when the feedstock is in solid form as the feedstock is also susceptible to air oxidation overtime.
Thus, there remains a recognized need for a compounded polymer, such as a compounded polyamide, such as compounded nylon66, that exhibits superior stability with improved performance with low variability of impurities, and the process for manufacturing such a product at the lowest possible cost. The compounded polymer prepared by the in-line process described herein shows improved molding performance with significantly lower plate out and longer mold life between cleanings compared to conventionally prepared compounded polymers where the feedstock is introduced to the polymer compounding equipment in solid form. The viscosity and quality control ranges for such a product are also less variable than that resulting from conventional pellet fed compounding. This corresponding improvement in quality directly translates into a superior product for the customer in terms of improved performance and lower cost compared to products prepared from a conventional compounding operations using feedstock in solid form.